Magnetron oscillator



Dec. 25, 1962 v. J. FOWLER MAGNETRON OSCILLATOR Filed April 15, 1960 itd This invention relates to electron tubes and in particular to an electron tube for use as a magnetron oscillator.

Magnetrons having negative-resistance characteristics have been used in the past for the generation of relatively large amounts of power at very high frequencies. One well known device of this type utilizes a split-anode con struction in which a cathode is surrounded by a cylindr'ical anode longitudinally divided into two or more segments. A resonant circuit is connected between the anode segments, this circuit usually being mounted external to the tube since the split-anode magnetron is seldom operated at frequencies exceeding 800 megacycles per second. Power outputs of several hundred watts and efficiencies as high as 50 to 60 percent have been obtained at UHF and VHF frequencies with this device.

However, the split-anode magnetron, as well as other radio frequency oscillators, has not proved entirely satisfactory for low voltage, low cost applications, such as electronic ovens suitable for home cooking. In general, these oscillators require the use of high-voltage power transformers and rectifying apparatus to convert the alternating current from the power main to a form suitable for driving the anode circuit of the tube. Further, the split-anode magnetron is inefiicient when operated at low plate-supply voltages, is comparatively large in physical size, requires a strong magnetic field, and must be cooled to reduce the anode temperature to an acceptable value. These disadvantages arise because a splitanode magnetron suitable for use at low voltages and high currents would necessarily have a small ratio of anode to cathode diameters. Consequently, its anode would be divided into longitudinal segments and the interaction space would take the form of a narrow cylindrical band, only a relatively small portion of the tube being occupied by the electron flow. Such a tube would be quite large in comparison with high-voltage tubes of the same power capacity. Furthermore, the magnetic field requirements for the tube would be great because the magnet must support an extensive fringe field inside of the cathode and outside of the anode.

Accordingly, it is an object of this invention to provide an improved oscillator capable of efficiently generating large amounts of radio frequency power.

Another object is to provide a generator of radio frequency power which can operate directly from a 220 v., 110 v. or other low voltage A.-C. power source.

Still another object is to provide a generator of radio frequency power which operates directly from low voltage A.-C. power mains without the use of auxiliary power supplies, rectifiers, transformers, or cooling equipment.

A further object of the invention is to provide an electron tube which requires a heater solely for initiating operation of the tube.

Still a further object of the invention is to provide a high frequency, high power electron tube which is simple in design, uses inexpensive materials and does not require precision manufacture.

Yet another object is to provide an electron tube of the magnetron type which requires only a relatively small and inexpensive magnet for efficient operation.

In the present invention, a magnetron oscillator is provided in which an array of conducting electrodes is positioned within an evacuated envelope. Some or all of gate Patent f Patented Dec. 25, 1$fi2 these electrodes are electron emissive; that is, they have been treated to provide copious electron emission when the cylinders are heated to a high temperature. The elec-. trodes are confined within a magnetic field having a direction substantially parallel to their longitudinal axes.

In one embodiment of the invention, the electrodes are divided into four electrically connected groups. A first resonant circuit is coupled between the first and fourth groups of electrodes while a second resonant circuit is coupled between the second and third group of electrodes. One terminal of a low frequency alternating voltage source is connected to the first and fourth groups of electrodes and the other terminal of the alternating voltage source is connected to the second and third groups of electrodes.

In operation, the emissive electrodes are heated sufiiciently to produce the electron emission required for converting the low frequency energy generated by the alternating voltage source to radio frequency energy. The potential of each electrode is the superposition of two nearly sinusoidal voltage components, one at the frequency of the alternating voltage source and the other at the radio frequency. During successive alternations of the alternating voltage source each emissive electrode functions alternately as a cathode and as an anode, the heated emissive electrodes liberating electrons during the negative half cycles of the A.-C. source and collecting electrons during the positive half cycles. Those cylinders which are incapable of liberating electrons function only to collect electrons during the positive half cycles of the alternating voltage source.

The electrodes are arranged within the evacuated envelope so that each of the electrodes in the first and fourth groups i in close proximity to one or more electron emissive electrodes comprising the second and third groups. The electrodes capable of collecting the electrons emitted by a given emitting cylinder are not, in general, all at the same R-F polarity. Therefore, as will be described hereinafter, the emitting electrodes supply large electron currents to adjacent electrodes which are at low positive potentials relative to the emitting electrode and smaller electron current to adjacent cylinders which are at high positive potentials relative to the emitting cylinder.

A heater is provided for at least one of the emissive cylinders to permit starting the oscillator from its cold condition. This heater is automatically disconnected when the tube reaches operating temperature. Since no heater power is required during normal operation, the efficiency of the device is high and no external cooling is required. Also, the electrode configuration permits use of a relatively small magnet and operation at low voltages.

In another embodiment of the invention the electrodes in the first and fourth groups are connected together and to on terminal of an alternating voltage source. A resonant circuit is connected between the electrodes in the second and third groups, and the other terminal of the alternating voltage source is conductively coupled to the second and third groups of electrodes. Thus, only one resonant circuit is employed in this form of the invention.

The above objects of and the brief introduction to the present invention will be more fully understood and fur-' FIG. 3 is another embodiment of the invention using only a single resonant circuit; and

FIG. 4a is a plan view and FIG. 4b is a reduced elevation view of a preferred form of the tube using a multi cylinder electrode matrix.

Referring to FIG. 1, there is shown an evacuated envelope enclosing four cylindrical conducting electrodes A, B, C, and D. The electrode A, B, C, and D are connected to terminals 12, 14, 16, and 18, respectively. A magnetic field produced by a permanent magnet (not shown) is oriented along a line parallel to the longitudinal axis of cylinders A-D. A first resonant circuit in the form of a transmission line 21') is coupled between cylinders B and C, while a second transmission line 22 is connected between cylinder A and D. Both resonant circuits are tuned to the desired radio frequency. An alternating voltage source 2 5 is connected between adjustable shorting bars a and 22a of transmission lines 29 and 22. Source 24 may be a 220 volt 6O cycle'power main of the type available in many homes for the operation of cooking and heating appliances. While transmission lines 2% and 22 have been employed as resonant circuits in the negative resistance oscillator of FIG. 1, it shall be understood that when the oscillator is used in conjunction with a cooking oven, the oven enclosure may comprise the resonant cavity.

The phase relationship existing between the low frequency alternating voltage generated by voltage source 24 and the radio frequency voltage appearing across the transmission lines 20 and 22 is shown by the polarity marks on generator 24 and on each of the terminals. The circled polarity marks indicate the instantaneous polarity of the low frequency voltage source 24, while the uncircled polarity marks correspond to the polarity of the generated radio frequency voltage. Thus, it will be noted that at the instant of time depicted cylinder B and C are at a positive low frequency potential and cylinders A and D are at a negative low frequency potential. Simultaneously, cylinders A and B are at positive R-F potentials and cylinders C and D are at negative R-F potentials. In some applications it may be desirable to connect a first capacitor between terminals 12 and 14 and a second capacitor between terminals 16 and 18 to adjust the phase relationship between the radio frequency voltages appearing across transmission lines 20 and 22.

In a preferred arrangement of FIG. 1, all of the cylinders A-D are electron emissive when heated to high temperatures. In order to start the oscillation from a cold condition, a heater is provided in cylinder A. Heater 30 is energized through a thermal relay (not shown) which automatically disconnects the heater from its power source when the cylinders reach operating temperature.

In FIG. 2 there is shown an enlarged view of the cylinders A-D, the electric fields existing between cylinders A, B, and C being indicated by electric field lines 32. The instantaneous low and high frequency components of potentials of each of cylinders A-D are shown in FIG. 2 by polarity marks corresponding to those used in FIG. 1. A low frequency electric field exists between cylinder A and cylinder B and C, since, as shown in FIG. 1, cylinder A is connected to the negative terminal of low fre-' I quency source 24 and cylinders B and C are connected to the positive terminal of source 24. However, since cylinder C is at a negative R-F potential with respect to the R-F potential of cylinder B, the total potential of cylinder C with respect to cylinder A is lower than that of cylinder B with respect to cylinder A.

Dashed line 40 is an equipotential line indicating the voltage distribution between the cylinders A, B and C. Since cylinder C is at a lower potential than cylinder B, the equipotential line 40 comes closer to it than it does to cylinder B. Electrons emitted from cylinder A tend to follow a trochoidal path consisting of a circular motion at the cyclotron frequency and a drift motion of the center of the generating circle along the equipotential line. Thus, an electron e emitted from the strong field region on the right side of cylinder A makes a looping trajectory 42 along the equipotential line 49 under the influence of the electric and magnetic fields and impinges on cylinder C. A second electron e emitted from the weaker field region at the bottom of cylinder A makes only a single loop and returns to cylinder A because the equipotential line 44 about which it tends to loop approaches closer to cylinder A than to cylinder C. There is, therefore, a strong tendency for electrons to be collected on the lower potential cylinder C and to avoid being collected on the higher potential cylinder B. This produces a negative resistance effect which is evidenced by an incremental increase in current for each incremental decrease in cylinder voltage over the operating region of the tube.

In FIG. 3 there is shown a modified version of this de vice in which the electrodes A and D are coupled together by an internal lead 50 and only a single resonant circuit 20 is employed. The operation of this tube is similar to that already described with respect to FIG. 1 and therefore will not be repeated. It will be noted, however, that since only one resonant circuit is employed the possibility of oscillations occurring at more than one frequency is minimized.

FIGS. 4a and 4b depict schematically plan and elevation views of a preferred form of tube for use with the invention. In this embodiment, a plurality of right circular, conductive, electron emissive cylinders are disposed in a rectangular matrix array within an evacuated envelope 6i). The cylinders are divided into four groups, each group forming one composite electrode. The six cylinders designated by the letter A are connected electrically together and to pin 62 of the tube. Similarly, the four B cylinders are connected together and to pin 64, the siX C cylinders are connected together and to pin 66, and the four D cylinders are connected together and to pin 68. The interconnections are made by means of jumper wires welded between the ends of the cylinders in each group.

A magnetic field having a direction parallel to the longitudinal axis of cylinders A-D is produced by a U- shaped magnet 70 located outside the envelope. Magnet 7t), which may be either an electromagnet or a permanent magnet, is shown in FIG. 41) but has been omitted from FIG. 4a for greater clarity. A heater 72, energized through a thermal relay (not shown) is provided in one of the cylinders 74 located near the center of the tube. In starting the tube, the heated cylinder '74 supplies electrons which bombard four of the neighboring cylinders. These cylinders heat up in turn and supply electrons which bombard other cylinders in the matrix. Thus, eventually all of the cylinders are heated. This arrang ment of electron emitters efiiciently conserves heat thereby enabling a large total cathode area to be heated with a relatively small amount of power. The various cylinders reflect each others radiant heat and in effect keep each other warm.

The tube may be employed as a magnetron oscillator by connecting a first resonant current between pins 62 and 68 and a second resonant circuit between pins and 66. One terminal of a low frequency voltage source is coupled to pins 62 and 63 and the other terminal to pins 64 and 66 If the first and second resonant circuits are transmission lines, as shown in FIG. 1, the voltage source may be coupled to the transmission line shorting bars. Alternately, the oscillator tube may be mounted inside of a resonant cavity and the voltage source coupled to taps on a pair of center tapped coupling loops connected across pins 62, 68 and 64-, 66 respectively.

It is also possible to operate the tube as a negativeresistance oscillator by omitting the resonant circuit between pins 62 and 63 and connecting these pins to one terminal of a low frequency voltage source in the manner shown in FIG. 3.

A significant feature of this invention is that a multi electrode tube suitable for use in a magnetron oscillator is provided in which a plurality of essentially identical electron emissive cylinders act alternately as anodes and cathodes. in addition, except for starting, the tube does not ar Li require a heater and is, therefore, highly efiicient. The tube is simple in design, operates on low voltages, uses inexpensive materials, and does not require a strong magnetic field.

What is claimed is:

1. An electron tube comprising an envelope adapted for mounting within a magnetic field; an array of conducting substantially parallel electrodes having their longitudinal axes located parallel to said magnetic field disposed within said envelope, at least two of said electrodes being electron emissive; means for applying an alternating voltage having a first instantaneous polarity to selected electrodes of said array, and means for applying an alternating volt age of opposite polarity to the remainder of said electrodes in said array, said selected electrodes having said voltage of first polarity applied to them being disposed in said array so that each electrode is adjacent to at least one electron emissive electrode having an alternating voltage of opposite polarity applied to it, the space between said electrodes being evacuated.

2. An electron tube as defined in claim 1 wherein each of the electrodes in said array emits electrons when heated.

3. An electron tube as defined in claim 1 wherein at least one of said electrodes is provided with heating means, said heating means being energized by a source external to said envelope to initiate operation of said tube.

4. In combination with an evacuated envelope located in a magnetic field, an oscillator comprising first, second, third and fourth groups of conducting substantially cylindrical electrodes mounted within said evacuated envelope, the axis of said electrodes being parallel to the direction of said magnetic field, a first resonant circuit coupled between said first and fourth groups of electrodes, a second resonant circuit coupled between said second and third groups of electrodes, means for applying an alternating voltage having a first polarity to said first and fourth groups of electrodes, and means for applying a voltage of opposite polarity to said second and third groups of electrodes.

5. Magnetron apparatus comprising first, second, third and fourth substantially parallel electron emissive conductive cylinders, first and second resonant circuits coupled between said first and fourth conductive cylinders and said second and third conductive cylinders respectively, means for applying one terminal of an alternating voltage source to said first and fourth conductive cylinders and the other terminal of said alternating voltage source to said second and third conductive cylinders, and means for applying a magnetic field parallel to the axis of said conductive cylinders.

6. In combination an evacuated envelope located in a magnetic field, an oscillator comprising an array of first, second, third, and fourth groups of spaced conducting electrodes mounted 'within said envelope, said electrodes having their longitudinal axes positioned parallel to the direction of said magnetic field, each of the electrodes in said first and fourth groups being adjacent at least one electrode in said second group and at least one electrode in said third group; a resonant circuit coupled between said first and fourth groups of electrodes; and means for applying one terminal of an alternating voltage source to said first and fourth groups of electrodes and the other terminal of said voltage source to said second and third groups of electrodes.

7. The oscillator defined in claim 6 wherein a second resonant circuit is coupled between said second and third groups of electrodes.

8. In combination with an evacuated envelope located in a magnetic field, an oscillator comprising first, second, third, and fourth parallel conducting electron emissive cylinders mounted within said envelope, said cylinders having their longitudinal axes positioned parallel to the direction of said magnetic field, a resonant circuit coupled between said first and fourth conductive cylinders, and means for applying one terminal of an alternating voltage source to said first and fourth conducting cylinders and the other terminal of said voltage source to said second and third conductive cylinders.

9. An electron tube comprising an evacuated envelope; first, second, third and fourth groups of substantially parallel electron emissive conductive cylinders disposed within said evacuated electroluminescent envelope, said cylinders being dispersed in a rectangular matrix having at least first, second, third and fourth rows, the first and third rows of said matrix including cylinders of said first and second groups and the second and fourth rows of said matrix including cylinders of said third and fourth groups, the cylinders of said second group being positioned between cylinders of said first group, and the cylinders of said fourth group being positioned between cylinders of said third group; and first, second, third, and fourth terminals located external to said envelope, said first, second, third and fourth terminals being connected respectively to said first, second, third and fourth groups of cylinders respectively.

References Cited in the file of this patent UNITED STATES PATENTS 1,571,463 Zworykin Feb. 2, 1926 1,585,766 Chubb May 25, 1926 2,121,067 Brown et al. June 21, 1938 

